and in it, there's a subdirectory called 'chrome'. In it, create a file called 'userContent.css' (there's an example file included by default in most distributed versions of Firefox). Any CSS rules you want to apply, put in that file and restart Firefox. For example, to enforce a maximum width of 200px for all text, you could use this:

\r\n\r\n
p, blockquote, pre, ol, ul, dl, dd, dt, li, h1, h2, h3, h4, h5, h6, input, textarea, legend, select, q { max-width: 200px!important; }
\r\n\r\n

I think that'll cover all the commonly-used text elements. It may cause some problems with certain sites' layouts, but not as many problems as if the rule were broader.